Lesson 2 - Balancing Chemical Equations

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Title: Balancing Chemical Reactions

Summary

This is the second lesson in the unit on chemical reactions. The topics focused on in this lesson are expressing chemical reactions using chemical equations and balancing chemical equations. The purpose of this lesson is to learn how to convert between word equations and skeleton equations and how to balance skeleton equations by following the law of conservation of mass. The main idea is that students will be able to see that in order for an equation to be balanced, there must be the same amount of each type of atom on both sides of the chemical equation. Students will use their prior knowledge to convert between word and skeleton equations and then to write and balance chemical equations. This lesson will lead into the next lesson that includes different types of chemical reactions. It is important for students to have a strong understanding of chemical equations in order to build on it and apply it to understanding chemical reactions.

Objectives

Students will be able to:
  • distinguish between reactants and products in a chemical reaction
  • convert between word equations and skeleton equations by writing the correct chemical formulas
  • describe chemical reactions using skeleton equations, word equations, and balanced chemical equations
  • represent chemical equations visually by drawing each atom and balancing using this method.
  • write balanced chemical equations and balance skeleton equations

Materials

Instruction

Opening (10 minutes)

  • When students walk into the classroom, the objectives of the day and the Chemistry Checkpoint question will be posted on the board.
  • Chemistry Checkpoint: List 5 signs that a chemical reaction has taken place.
  • After students have completed the Chemistry Checkpoint question, I will quickly ask 5 students to share one of their answers. Then I will ask if anyone has anything different that they would like to add.
  • Students will turn in their Where's the Evidence? labs.


Middle (60 minutes)

  • We will take notes on how to represent chemical reactions using word equations and skeleton equations. I will describe each type of equation and show examples of each. On the board we will go through how to convert between word equations and skeleton equations using correct chemical formulas. Students previously learned how to write chemical formulas in the last chapter, so this will be a good review and practice.
  • After we have done a few examples on the board, I will explain that skeleton equations do not fully describe what is happening in a chemical reaction. In order to describe a chemical reaction completely, we must write balanced chemical equations.
  • I will show the combustion of methane overhead, which includes the skeleton equation and a visual representation of the reaction. Under the reactants I will list each type of atom (C=, H=, O= ) and I will draw a line down the center of the arrow and do the same for the products. I will ask the students to count up the number of each type of atom in both the products and the reactants.
  • I will ask students if it matters that there are not the same amount of each type of atoms on both sides of the equation. Do we have to have the same amount on both sides? Why? yes- the law of conservation of mass states that the mass of the products must equal the mass of reactants. If students do not remember this, I will quickly go over it.
  • I will explain to students that the magic of chemistry is that we can pull apart all the reactant pieces and rearrange them to form the products. The only requirement is that we end with the same amount of "pieces" that we started with. We don't get to make new and different products, we only get to change how much of each atom we have using coefficients. That's the balancing part. Atoms cannot be created or destroyed in a reaction. Scientists know that there must be the same number of atoms on each side of the equation. To balance the equations we must add coefficients in front of the chemical formulas; we cannot add or remove subscripts because that would change the chemical formula, and we are not allowed to make new products!
  • Next, I will show the rules for balncing chemical equations.
    • 1. Determine the number of atoms for each element in the products and the reactants.
    • 2. Pick an element that is not equal on both sides of the equation.
    • 3. Add a coefficient in front of the chemical formula with that element and adjust your counts.
    • 4. Continue adding coefficients in front of chemical formulas until the number of atoms for each element are equal on both sides.
  • We will follow these rules to balance the combustion of methane together as a class.
  • I will pass out Balancing Chemical Equations Practice Worksheet as well as a typed copy of the rules for balancing chemical equations.
  • I will have a student read the directions out loud. I will model how to complete the worksheet by doing the first problem step by step on the board. I will have the students guide me by asking them what I do at each step. If the students seem ready to begin working on their own, I will instruct them to get some markers/colored pencils and get to work on the rest of the worksheet. If they still seem unsure, we will go through another problem together.
  • Students will work on the worksheet for 20-25 minutes. If some students finish early I will provide them with a Challenge Problem. While students are working, I will be walking around the classroom and making sure students are staying on task. I will be available for students to ask questions and to guide students in the right direction. This will be good as an informal formative assessment because I will be able to see what students are finding the most difficult, and I will be able to see if anyone is having trouble.
  • The class will regroup and I will ask if anyone had difficulty with any of the problems. If students express a concern with any of the problems, I will put it on the board and ask for a volunteer to do it out on the board. If not, I will choose one to have someone do on the board in order to assess how students are doing with this task.
  • As a wrap up for this lesson, we will write the main idea of the lesson and why it is important as a class. I will write it on the board and they will write it in their daily log.

Closing (20 minutes)

  • The final 20 minutes of class will be dedicated to introducing the project for the unit. I will pass out the Adopt-a-Chemical Reaction project sheet as well as a rubric for grading.
  • I will go over the project by describing what they will be doing and what they are expected to include on their posters. i will also go over the rubric so students know exactly how they can earn/lose points on the project.
  • I will put the due dates for each section of the project on the board and tell them to write it on their assignment sheets.
  • 5-10 minutes will be given to students to pick their partners and begin thinking about what reaction they would like to research.
  • In the last 2 minutes of class, I will hand out the homework and tell students to bring their reaction proposals in for next class.

Notes